This application is related to and claims priority, under 35 U.S.C. xc2xa7119, from French Patent Application No. 99 02129, filed on Feb. 22, 1999, the entire contents of which are hereby incorporated by reference herein.
1. Field of the Invention
The present invention generally relates to a shadow mask support frame and more particularly, to a shadow mask support frame for a cathode-ray color display tube, which is both lightweight and rigid and easy to manufacture.
2. Discussion of Background
Cathode-ray color display tubes include a metal sheet pierced with a multitude of holes or slits, called xe2x80x9cshadow maskxe2x80x9d, placed between the electron gun and the display screen. This shadow mask is intended to obtain a very sharp image by ensuring that the impacts of the electron beams on the display screen are located precisely on the phosphors placed on the display screen.
The shadow masks are supported by frames of generally rectangular shape which hold them in position near the display screen and, optionally, ensure that they are tensioned so as to limit any deformation resulting from local heating generated by the electron beams.
According to a known technique, a shadow mask support frame has two lateral upstands, consisting of metal tubes or of angle sections, and two end upstands, consisting of angle sections or of tubes placed on the lateral upstands and joined together by welding at the points of contact. Given the method of construction, the tubes or angle sections must be relatively thick in order to obtain sufficient stiffness. The frames thus formed are suitable for tensioned shadow masks, but have the drawback of being heavy and difficult to manufacture with good geometrical precision.
According to another known technique, a shadow mask frame consists of angle sections produced from thin strips shaped so as to form angle sections which are joined together by welding. In a variation, the frame is produced by drawing a generally rectangular metal sheet having a central cut-out, which is also rectangular. The frames, produced as above-described, have the advantage of being lightweight, but have the disadvantage of not being very rigid and consequently, are poorly suited to supporting tensioned shadow masks.
In order to improve the stiffness of lightweight frames, it has been proposed, especially in French Patent Application Publication No. 2,749,104, to manufacture such frames from two thin metal strips placed against each other and to have vertical and horizontal stiffening ribs. These frames are both lightweight and rigid, allowing them to be used for tensioned shadow masks, but they have the drawback of sometimes being difficult to manufacture.
The object of the present invention is to remedy the above-stated drawbacks by providing a shadow mask support frame which is both lightweight and rigid and easy to manufacture.
For this purpose, the subject of the present invention is a shadow mask support frame for a cathode-ray color display tube, of generally rectangular shape, comprising two end upstands, lying opposite each other and intended to house a shadow mask, and two lateral upstands, intended to keep the end upstands apart. The end upstands and the lateral upstands are of generally tubular shape and constitute an almost continuous closed hollow body containing at least one plane closed line located entirely inside the hollow body and passing inside each of the end and lateral upstands. This frame consists of at least one thin metal membrane.
The frame may, for example, consist of two thin metal membranes drawn so as to form an external shell and an internal shell, the two shells being fitted, one in the other, so as to form a hollow body.
The two shells may be made either of the same material or of two different materials.
The frame may also consist either of a single metal membrane folded so as to form a hollow body, or of several metal membranes folded so that each forms a hollow body constituting a frame portion, the frame portions then being joined together.
The end upstands, intended to house a shadow mask, may have a generally triangular cross section and include a flap along the upper arris, to which flap the edge of a shadow mask may be welded. They may also have a generally trapezoidal cross section so as to have an upper arris wide enough to allow welding of an edge of a shadow mask. The upper arris may be rectilinear or curvilinear.
The lateral upstands may have a cross section of any shape, and in particular a rectangular shape, or preferably a trapezoidal shape.
The frame may more particularly be intended to house a tensioned shadow mask.
The metal membrane or membranes of which the frame is composed are made of a metallic material taken, preferably, from the following materials: low-carbon steels, iron-based alloys, iron-nickel alloys, iron-chromium alloys, nickel-based alloys, structural-hardening alloys, martensitic-type hardening alloys, magnetic alloys, non-magnetic alloys and vibration-damping alloys. When the frame is made of several materials, these may be chosen, for example, so that the thermal expansion coefficient of the lateral upstands is different from the thermal expansion coefficient of the end upstands.
The invention also relates to a process for manufacturing a frame, in which a strip having the developed shape of the frame or of the frame portion or a plurality of strips having the developed shape of a plurality of complementary frame portions is cut from a sheet of metallic material, the strip or strips are folded along a plurality of lines so as to form the frame or the frame portions and, when the frame is made from a single piece, the edges of the strip which are superimposed one with respect to the other are fastened together by welding, brazing or clinching. When the frame is made from several pieces, for each of the frame portions, the edges of the strip which are superimposed one with respect to the other are fastened together by welding, brazing or clinching, and the frame portions are joined together by fastening them together by welding, brazing or clinching.
Preferably, the welding is laser welding or electron-beam welding. The shadow mask support frames, obtained by either laser or electron-beam welding, are both very rigid and very lightweight. Furthermore, they are easy to manufacture with a very high geometrical precision. In addition, they exhibit very good vibration behavior and in particular, significant damping.